1. Field of the Invention
The present invention relates to the drive circuit for a piezoelectric transformer that generates an ac voltage using a piezoelectric transformer element, and particularly relates to a drive method during a state in which load is open or in which a cold cathode tube used as load has high impedance and does not turn on.
2. Description of the Related Art
A piezoelectric transformer is generally an element in which primary and secondary electrodes are affixed to a piezoelectric material, the transformer is caused to mechanically resonate by impressing voltage of the transformer resonance frequency on the primary side, and the voltage generated due to this mechanical resonance is extracted from the secondary side. Such an element features greater capabilities for production in smaller and thinner sizes than an electromagnetic transformer and is therefore receiving attention for such applications as the backlight power source in liquid crystal displays.
Japanese Patent Application 264081/95 discloses one example of this type of piezoelectric element drive circuit. As shown in FIG. 1, this circuit is made up of: piezoelectric transformer 1 that inputs an ac voltage from primary electrodes and outputs from secondary electrodes using the piezoelectric effect; first auto transformer 5 that has its secondary terminal connected to one of the primary electrodes of piezoelectric transformer 1 and its primary terminal connected to the power source; first switching transistor 7 that has its output terminal connected to an intermediate terminal of first autotransformer 5; second autotransformer 6 that has its secondary terminal connected to the other of the primary electrodes of piezoelectric transformer 1 and its primary terminal connected to the power source; second switching transistor 8 that has its output terminal connected to an intermediate terminal of second autotransformer 6; frequency divider circuit 9 that alternately drives first switching transistor 7 and second switching transistor 8; frequency control circuit 3 that outputs a drive signal and a triangular wave signal to frequency divider circuit 9 and drive voltage control circuit 11, respectively; drive voltage control circuit 11 that controls the peak voltage that drives the piezoelectric transformer to a fixed level; dimmer circuit 12 that generates a drive halt signal to drive voltage control circuit 11 and performs drive ON/OFF duty control and in addition, outputs a control signal to frequency control circuit 3 such that the frequency of VCO 18 (FIG. 12) does not vary between drive halts.
This construction enables a fixed voltage and current to be outputted to a load despite fluctuation in power source voltage VDD, and in addition, enables prevention of drops in conversion efficiency because the frequency driving the piezoelectric transformer does not vary.
Next, regarding the operation of this circuit, first switching transistor 7 and second switching transistor 8 alternately turn ON in response to clocks of opposite phases outputted from frequency divider circuit 9, current flows from power source VDD to the primary sides of first autotransformer 5 and second autotransformer 6, and this current energy is charged on the autotransformer windings as current energy. When first switching transistor 7 and second switching transistor 8 turn OFF, the charged energy is discharged and a voltage higher than the power source voltage is generated as voltage energy.
FIG. 18 shows the piezoelectric transformer input voltage waveforms Vs1 and Vs2 and the switching transistor drain voltage waveforms Vd1 and Vd2. These waveforms are voltage-resonated waveforms due to the equivalent input capacitance of piezoelectric transformer 1 and load 2 and the sum inductance of the primary inductance and secondary inductance of an electromagnetic transformer, and are set so as to become the half-wave of the sine wave that reaches zero voltage in the time interval of one-half the resonance period of piezoelectric transformer 1.
The use of autotransformers has the advantage of allowing a higher boost ratio than a two-winding electromagnetic transformer of the same turn ratio, and moreover, because the same boost ratio can be obtained with a lower turn ratio, has the advantage of making an electromagnetic transformer smaller and thinner.
The generated voltage is alternately inputted to the primary electrodes of piezoelectric transformer 1; therefore a drive voltage of the waveform equivalent to a sine wave oscillates piezoelectric transformer 1, and an output voltage of the boost ratio times, which is determined by the form of piezoelectric transformer 1, is outputted from the secondary electrodes. This voltage V.sub.0 is impressed to load 2, and current I.sub.0 that flows through load 2 and is fed back is inputted to frequency control circuit 3. Frequency control circuit 3 generates the frequency that drives piezoelectric transformer 1 via frequency divider circuit 9, continues sweeping the drive frequency until the feedback current I.sub.0 from the load reaches a prescribed value, and holds the frequency at a frequency at which the prescribed value is obtained.
As shown in FIG. 14, frequency control circuit 3 is made up of voltage-current conversion circuit 13, rectifying circuit 14, comparator 15, integrator 16, comparator 17, and VCO 18. Current I.sub.0 which is fed back from load 2 is converted at current-voltage conversion circuit 13, converted to dc voltage at rectifying circuit 14, and inputted to comparator 15. At comparator 15, the inputted voltage is compared with reference voltage Vref and a high-level signal is outputted to integrator 16 if the inputted voltage is lower. Integrator 16 is constructed so as to boost the input voltage by a fixed ratio during the interval of input of high-level voltage, and this output voltage is inputted to VCO 18. VCO 18 is a voltage-controlled oscillator that outputs a triangular wave voltage at a frequency in inverse proportion to the inputted voltage. The oscillation frequency of VCO 18 is frequency-divided at frequency divider circuit 9, and piezoelectric transformer 1 is driven at this frequency. Accordingly, the drive frequency continues to drop if a voltage lower than the reference voltage Vref is inputted to comparator 15.
The drive frequency of piezoelectric transformer 1 is set so as to fall from f1 shown in FIG. 13, and accordingly, approaches the resonance frequency fr having the highest boost ratio of piezoelectric transformer 1. The boost ratio of piezoelectric transformer 1 therefore increases and the output current of piezoelectric transformer 1 increases over time. When the voltage inputted to comparator 15 becomes greater than the reference voltage Vref at drive frequency f0, the output voltage of comparator 15 becomes low level. With this signal, the output signal of integrator 16 is held unchanged at the voltage immediately preceding the change to low level, the output frequency of VCO 18 is held uniform, and piezoelectric transformer 1 is driven at a fixed frequency.
In a case in which load 2 is a cold cathode tube and the tube current is not sufficient to turn on the cold cathode tube despite the activation of power source VDD or when the voltage inputted to booster circuit 4 is low, a state exists in which feedback current I.sub.0 is not generated such that the input voltage to comparator 15 is greater than the reference voltage Vref. Under these conditions, the output of comparator 15 remains unchanged at high level and the drive frequency continues to drop. Upon reaching frequency f2 shown in FIG. 13, comparator 17, which inputs the output of integrator 16, exceeds its reference voltage Vmin and outputs a high-level signal to reset integrator 16. Integrator 16 is thereby reset, the output voltage becomes the minimum voltage, and the output of VCO 18 enters a state in which frequency divider circuit 9 outputs frequency f1. The drive frequency drops from f1, and the above-described operation is repeated.
As shown in FIG. 16, drive voltage control circuit 11 is a circuit in which the drain voltage waveform Vd1 of first switching transistor 7 is voltage-divided and rectified at voltage divider/rectifier circuit 20, inputted to the non-inverted input terminal of comparator 19, the triangular wave Vr of the drive frequency generated at VCO 18 is inputted to the inverted input terminal, and the comparison result is inverted and inputted to the gate of a p-channel power MOSFET (hereinbelow referred to as "Q3") by way of an OR circuit. FIG. 19 is a timing chart showing the operation of: voltage Vc, which is the result of voltage-dividing and rectifying drain voltage waveform Vd1; triangular wave Vr of the drive frequency generated by frequency control circuit 3; Q3 gate voltage Vg3; output voltages Vg1 and Vg2 of the frequency divider circuit; first switching transistor drain voltage waveform Vd1; and second switching transistor drain voltage waveform Vd2. When the drain voltage is high, the non-inverted input terminal voltage Vc becomes large, and the higher the voltage Vc inputted to comparator 19, the greater the proportion of time during which Q3 gate voltage Vg3 is outputted. The time interval during which the source and drain of Q3 is open thus increases and the input power to booster circuit 4 decreases, and control is therefore executed to lower the switching transistor drain voltages Vd1 and Vd2. When the drain voltages are low, non-inverted input terminal voltage Vc becomes low, and the lower the voltage Vc inputted to comparator 19, the smaller the proportion of time during which Q3 gate voltage Vg3 is outputted. Thus, the time interval during which the source-drain of Q3 is open becomes shorter, the input power to booster circuit 4 increases, and control is executed to raise the switching transistor drain voltages Vd1 and Vd2. Switching transistor drain voltages Vd1 and Vd2 are therefore controlled to fixed voltage values by this continuous control, and a means of maintaining a constant voltage for driving a piezoelectric transformer is realized.
As shown in FIG. 22, dimmer circuit 12 is made up of a triangular wave oscillation circuit 27 that oscillates a dimmer frequency, dc voltage generation circuit 33, and comparator 29. The dc voltage V.sub.b generated at dc voltage generation circuit 33 and the output waveform of triangular wave oscillation circuit 27 are compared at comparator 29, and a variable duty pulse signal is outputted. This signal is connected to frequency control circuit 3 and drive voltage control circuit 11, and serves to turn OFF Q3 during the interval of high level and halt the drive of piezoelectric transformer 1, as well as to hold the output voltage of integrator 16 such that the frequency of VCO 18 does not vary.
When the output voltage of piezoelectric transformer 1 is excessive, the piezoelectric transformer 1 itself breaks down. Japanese Patent Laid-open No. 107678/96 discloses a drive circuit for preventing such breakdown. According to this invention, as shown in FIG. 2, output voltage comparison circuit 10 is connected to the secondary electrodes of piezoelectric transformer 1, and output voltage comparison circuit 10 confers the results of judgment to frequency sweep oscillator 22. Output voltage comparison circuit 10 voltage-divides and rectifies the voltage outputted to the secondary electrodes of piezoelectric transformer 1, has the function of judging whether or not the voltage outputted to the secondary electrodes of piezoelectric transformer 1 exceeds a preset output voltage by comparing the voltage-divided and rectified voltage with an internal reference voltage, and confers this judgment to frequency sweep oscillator 22. Frequency sweep oscillator 22 has the function of switching the frequency sweep and reverses the direction of frequency sweep from the direction of decreasing frequency to the direction of increasing frequency when it is determined that the judgment result of output voltage comparison circuit 10 exceeds the preset output voltage. By means of this function, the drive frequency of piezoelectric transformer 1 is shifted to a low boost ratio state and the output voltage is reduced when the load for whatever reason becomes open, thereby preventing breakdown of the piezoelectric transformer itself brought about by the state of excess oscillation due to the sudden rise in output voltage of a piezoelectric transformer that accompanies an abrupt increase in load impedance.
Nevertheless, the prior art according to Japanese Patent Application No. 264081/95 and Japanese Patent Laid-open No. 107678/96 has the following drawbacks. First, in a state resulting from, for example, low ambient temperature in which the impedance of the cold cathode tube is high and the tube does not turn on, or during a load-open state due to, for example, disconnection, the peak value of the primary current of an electromagnetic transformer may increase at the time of reversal of the frequency divider circuit, thereby giving rise to heating of first autotransformer 5, first switching transistor 7, second autotransformer 6, and second switching transistor 8.
This heating is caused by the increase of the peak value of the current flowing at the time of reversal of the frequency divider circuit to the level of the drive frequency, and under the above-described conditions, the drive frequency sweep will continue endlessly or for an extended period of time.
If the cold cathode tube has high impedance and does not turn on due to, for example, low ambient temperature, or during a load-open state due to, for example, disconnection, and a feedback current I.sub.0 is not generated such that the input voltage of comparator 15 exceeds the reference voltage Vref, the output of comparator 15 remains unchanged at high level and the drive frequency drops. The output impedance of piezoelectric transformer 1 is high and the magnitude of its output voltage depends on load impedance, and as a result, because load impedance is high in the above-described states, the output voltage of piezoelectric transformer 1 is also high. Output voltage comparison circuit 10 outputs at frequency f3 shown in FIG. 13. Integrator 16 is reset, the output voltage becomes the minimum voltage, and the output of VCO 18 enters a state such that frequency divider circuit 9 outputs frequency f1. The drive frequency drops from f1, and the above-described series of operations is repeated either endlessly or for an extended period of time.
The instant that a drive frequency f1 is outputted, switching transistor drain voltage waveform Vd1 takes on the waveform shown in FIG. 26, and Vd2 also takes on the same voltage waveform.
At frequency f0 shown in FIG. 13, Vd1 and Vd2 voltage-resonate in accordance with the equivalent input capacitance of piezoelectric transformer 1 and load 2 and the sum inductance of the primary inductance and secondary inductance of an electromagnetic transformer, and these voltage waveforms Vd1 and Vd2 are set to become the half-wave of a sine wave that reaches zero voltage in the time interval of one-half the resonance period of piezoelectric transformer 1. As a result, the time of one cycle decreases to the extent that the drive frequency increases over the resonance frequency f0 as shown in FIG. 26, and the voltage immediately preceding the switch to zero rises. An excessive current therefore flows at the instant of switching to zero, and accordingly, the higher the drive frequency, the greater the increase of the peak value of the current flowing to first autotransformer 5, first switching transistor 7, second autotransformer 6, and second switching transistor 8 at the instant of inversion of frequency divider circuit 9. In a case in which the sweep of the drive frequency is repeated endlessly or for an extended period of time, the drive frequency repeatedly passes the vicinity of frequency f1, which is far higher than f0, and the circuit components generate heat.
The second drawback of the prior art is the difficulty of design for shifting to a low frequency the setting of drive frequency f1 at which integrator 16 is reset and the output voltage reaches the minimum voltage. The reason for this is that, even if the feedback current I.sub.0 is set to the same value, the value of the drive frequency f0 that flows with the feedback current I.sub.0 stabilized will differ due to the changes in impedance brought about by the type of load, the environment in which the circuit is used, or the passage of time. In addition, because the set value of the feedback current I.sub.0 is changed by the setting level of the luminance of the cold cathode tube, which is the load, the drive frequency will vary despite the load impedance being fixed. In regard to the above-described points, the drive frequencies for all of the above-described conditions must fall within the drive frequency sweep range generated by VCO 18.
Regarding the operation of frequency control circuit 3, the frequency is progressively lowered from frequency f1, at which the output voltage reaches the minimum voltage with integrator 16 having been reset. When the voltage inputted to comparator 15 becomes greater than the reference voltage Vref, the output frequency is fixed and piezoelectric transformer 1 is driven at a fixed frequency. Accordingly, the tube will not turn on with stability unless frequency f1 is set to a frequency that is greater than the drive frequencies, which vary due to impedance of the load or luminance setting described hereinabove.
The solution to the first drawback described hereinabove is to shift f1 to a low frequency, while the solution to the second drawback is to shift f1 to a high frequency to increase the margin, and design of the device has therefore been complicated by these mutually contradictory solutions.